Method and apparatus for automatic registration for a board

ABSTRACT

An object of the present invention is to provide a high-precision inspection apparatus and inspection method capable of inspecting a large board by laterally moving the board and photographing the board several times before and after the lateral moving. Coordinate positions of correction marks applied to a pair of holding members or a pair of rails are read by a line sensor camera, an amount of positional discrepancy of the board is computed by comparing coordinate positions of correction marks previously set with the coordinate positions of the correction marks having been read, and when there are discrepancies between the two kinds of coordinate positions, photographing is carried out after correcting an amount of movement of the holding members based on the amount of positional discrepancy of the board.

FIELD OF THE INVENTION

[0001] This invention relates to an inspection method and inspectionapparatus wherein a board that has been conveyed to the inspectionposition is photographed with a line sensor camera and inspected.

BACKGROUND OF THE INVENTION

[0002] When electronic components are mounted to printed circuit boards,various kinds of inspections are performed. These include inspections ofprint conditions after solder printing, inspections of mountingconditions of mounted components, and inspections of conditions ofcomponents and solder after the solder has hardened.

[0003] One method for performing such inspections is the method ofinspecting by using a line sensor camera to photograph boards that havebeen conveyed to an inspection position.

[0004] A conventional inspection apparatus is shown in FIGS. 7 to 10.

[0005] As shown in FIGS. 7 and 8, the inspection apparatus is configuredsuch that an inspection is made by causing a line sensor camera 3 tophotograph a printed circuit board 1 conveyed to an inspection position,by scanning that printed circuit board 1 in a direction (directionindicated by arrow C) along the direction of board conveyance.

[0006] More specifically, the printed circuit board 1 in the inspectionposition is held by a pair of rails 2 being constant in breadth andconstituting a pair of holding members. The line sensor camera 3 ismovable in the direction of arrow C by a movement shaft 8, andphotographs the surface of the printed circuit board 1 through areflecting mirror 10.

[0007] In an inspection apparatus configured in this way, the movementshaft 8 is sometimes caused to expand and contract or the printedcircuit board 1 to expand and contract due to factors such as heat.

[0008] For this reason, in the printed circuit board 1 being inspected,as shown in FIG. 9, a first correction mark 6 e and a second correctionmark 6 f are provided on the side where the line sensor camera 3 startsreading and on the side where it ends reading, respectively, such thatthese correction marks 6 e and 6 f are positioned on a diagonal linepassing through the board 1. Using the correction marks in these twoplaces, any discrepancy in the movement shaft 8 or printed circuit board1 described above is corrected.

[0009] In more specific terms, when the printed circuit board 1 isphotographed by the line sensor camera 3, the correction marks 6 e and 6f are first recognized with the line sensor camera 3 and centerpositions therefor are found respectively. Coordinate positions for thecorrection marks 6 e and 6 f thus obtained by the recognition with theline sensor camera 3 are defined as (x_(e), y_(e)) and (X_(f), y_(f))respectively.

[0010] Here, coordinate positions for the first and second correctionmarks 6 e and 6 f have been pre-taught as (X_(e), Y_(e)) and (X_(f),Y_(f)), respectively, and distances between the first correction mark 6e and the second correction mark 6 f in the X axis direction and the Yaxis direction have been known, so that based on the distances and therecognition with the line sensor camera, the ratios of the distancesbetween the first and second correction marks 6 e and 6 f in the X axisdirection and the Y axis direction are found, respectively, as γ_(x),and γ_(y), that may be expressed as;

γ_(x) =|x _(e) −x _(f) |/|X _(e) −X _(f)| and γ_(y) =|y _(e) −y _(f)|/|Y _(e) −Y _(f)|.

[0011] When the ratios in the X axis direction and the Y axis directionare both valued 1, it can be judged that there is no expansion orcontraction in the movement shaft 8 or the printed circuit board 1. Whenthe ratio is other than 1, however, it is judged that there is expansionor contraction in the movement shaft 8 and/or printed circuit board 1,whereupon adjustments are made to respective coordinate positions in animage so that the recognized positions of the correction marks becomethe taught values, and the position of the printed circuit board 1 isalso adjusted.

[0012] More specifically, assuming that the expansion and contractionare uniform, respective X-coordinate and Y-coordinate values in therecognized image are divided by γ_(x) and γ_(y) to produce correctedvalues so that a corrected image can be presented.

[0013] However, as will be explained below, there is a problem with theinspection method described in the foregoing in that it cannot be usedwhen the printed circuit board 1 is so large in the breadth direction ofthe rails 2 that it cannot be thoroughly scanned by the line censorcamera.

[0014] When the printed circuit board 1 is too large as mentioned above,one possibility is to provide the printed circuit board 1 with a screwshaft that is movable in the breadth direction of the rails 2. Byrotating the screw shaft, the printed circuit board 1 is moved in thebreadth direction of the rails 2 a while photographs are taken with theline sensor camera 3 a plural number of times.

[0015] When the printed circuit board 1 is photographed two times withthe line sensor camera 3, for example, first, as shown in FIG. 10(a-1),the line sensor camera 3 is moved from a first scan start position 5 cin the direction indicated by arrow E1, whereupon a first scan area 7 awherein a first inspection point 11 a is provided is photographed.

[0016] Next, as shown in FIG. 10(a-2), the printed circuit board 1 ismoved in the direction indicated by arrow E2 by the screw shaft, and asecond scan area 7 b wherein a second inspection point 11 b is providedis photographed from a second scan start position 5 d.

[0017] The printed circuit board 1, after photographing is concluded, ismoved in the direction indicated by arrow F by the screw shaft, as shownin FIG. 10(a-3), and returned to the original position.

[0018] When there is contained a process step for moving the printedcircuit board 1 by a screw shaft in the breadth direction of the rails2, as described above, there is a problem in that when printed circuitboards 1 are conveyed successively and each printed circuit boardundergoes inspection, even if the position of the printed circuit board1 is altered by rotating the screw shaft for the same amount each time,the resulting amount of movement will not be constant in actual practicebecause of the influence of the thermal expansion of the screw shaft.

[0019] FIGS. 10(b-1) to 10(b-3) show the inspection method for a casewhere there has been thermal expansion in the screw shaft.

[0020]FIG. 10(b-1), where photographing is done in the same manner asshown in FIG. 10(a-1), shows a reference position for moving the printedcircuit board 1 with the screw shaft, so that the first scan startposition 5 c and the first scan start position 5 e are at the sameposition, and there is no discrepancy between the first scan area 7 aand the first scan area 7 c photographed by the line sensor camera 3.

[0021] However, when thermal expansion occurs in the screw shaft, themoved printed circuit board 1, as shown in FIG. 10(b-2), even though thescrew shaft is rotated for the same amount as shown in FIG. 10(a-2),exhibits a discrepancy between the second scan start position 5 f andthe second scan start position 5 d in the breadth direction of the rails2.

[0022] As a result, the distance from the second scan start position 5 dto the second correction mark 6 f as shown in FIG. 10(a-2) differs fromthe distance from the second scan start position 5 f to the secondcorrection mark 6 h as shown in FIG. 10(b-2). Likewise, the distancefrom the second scan start position 5 d to the second inspection point11 b as shown in FIG. 10(a-2) differs from the distance from the secondscan start position 5 f to the second inspection point 11 d as shown inFIG. 10(b-2). Accordingly, overall dislocation has occurred between theimage including the first scan area 7 c and the second scan area 7 d asshown in FIG. 10(b-3), and the image including the first scan area 7 sand the second scan area 7 b as shown in FIG. 10(a-3).

[0023] Thus, when a discrepancy in the rotational driving amount of thescrew shaft occurs due to heat or the like, a discrepancy occurs in thescan start position, so that a contraction correction method that uses afirst correction mark 6 c and a second correction mark 6 f, as describedin the foregoing, cannot be employed.

SUMMARY OF THE INVENTION

[0024] An object of the present invention is to provide a high-precisioninspection apparatus and inspection method capable of inspecting a largeboard by laterally moving the board and photographing the board for aplural number of times before and after the lateral moving.

[0025] The inspection method of the present invention is characterizedin that a to-be-inspected board is moved laterally and an amount ofmovement of holding members of the board is corrected based on an amountof positional discrepancy of the board.

[0026] According to this aspect of the invention, large boards can beinspected, and high-precision inspections can be effected.

[0027] The inspection apparatus of the invention is characterized inthat correction marks are provided on the holding members for holdingthe board.

[0028] According to this aspect of the invention, the inspection methodof the present invention can be easily implemented.

[0029] The inspection method described in claim 1 of the presentinvention is a method for inspecting a to-be-inspected board byphotographing the board that has been conveyed to an inspectionposition, in which a pair of holding members constituted of a pair ofrails for holding the board are moved to move the board in a directioncrossing a scanning direction of a photography unit and the board thathas been positioned is scanned by the photography unit along thedirection of said conveyance of the board, the inspection methodcomprising: reading coordinate positions of correction marks applied tosaid holding members by said photography unit; computing an amount ofpositional discrepancy of the board by comparing coordinate positions ofcorrection marks previously set with the coordinate positions ofcorrection marks having been read; and when there are discrepanciesbetween said two kinds of coordinate positions, carrying out saidphotographing after correcting an amount of movement of the holdingmembers based on said amount of positional discrepancy of the board.

[0030] The inspection method according to claim 2 of the presentinvention is a method for inspecting a to-be-inspected board byphotographing the board that has been conveyed to an inspectionposition, in which a pair of holding members constituted of a pair ofrails located in an opposed relation to each other for holding the boardare moved to move the board in a direction crossing a scanning directionof a photography unit and the board that has been positioned is scannedby the photography unit along the direction of said conveyance of theboard, the inspection method comprising: reading a coordinate positionof a first correction mark applied to one of the holding members by saidphotography unit; moving said holding members and reading a coordinateposition of a second correction mark applied to the other of the holdingmembers, said second correction mark on said the other of the holdingmembers being on the side opposite to that of the first correction markon said one of the holding members with the board being interposedtherebetween; computing an amount of positional discrepancy of the boardby comparing coordinate positions of first and second correction markspreviously set with the coordinate positions of the first and secondcorrection marks having been read; and when there are discrepanciesbetween said two kinds of coordinate positions, carrying out saidphotographing after correcting an amount of movement of the holdingmembers based on said amount of positional discrepancy of the board.

[0031] The inspection method described in claim 3 of the presentinvention is a method for inspecting a to-be-inspected board byphotographing the board that has been conveyed to an inspectionposition, in which a pair of holding members constituted of a pair ofrails for holding the board are moved to move the board in a directioncrossing a scanning direction of a photography unit and the board thathas been positioned is scanned by the photography unit along thedirection of said conveyance of the board, the inspection methodcomprising: reading coordinate positions of correction marks applied tosaid board by said photography unit; computing an amount of positionaldiscrepancy of said board by comparing coordinate positions ofcorrection marks previously set with the coordinate positions of thecorrection marks having been read; and when there are discrepanciesbetween said two kinds of coordinate positions, carrying out saidphotographing after correcting an amount of movement of the holdingmembers based on said amount of positional discrepancy of the board.

[0032] The inspection method described in claim 4 of the presentinvention is a method according to claim 2, wherein the first and secondcorrection marks are provided on the side where the photography unitstarts reading.

[0033] The inspection method described in claim 5 of the presentinvention is a method according to claim 3, wherein a first correctionmark and a second correction mark are provided on the board, said firstcorrection mark being provided on the side where the photography unitstarts reading, said second correction mark being provided on a diagonalline passing said first correction mark.

[0034] The inspection apparatus described in claim 6 of the presentinvention is an apparatus for inspecting a to-be-inspected board byphotographing the board that has been conveyed to an inspectionposition, in which a pair of holding members constituted of a pair ofrails for holding the board are moved to move the board in a directioncrossing a scanning direction of a photography unit and the board thathas been positioned is scanned by the photography unit along thedirection of said conveyance of the board, the inspection apparatuscomprising: a control unit configured to read coordinate positions ofcorrection marks applied to the holding members by the photography unit,to compute an amount of positional discrepancy of the board by comparingcoordinate positions of correction marks previously set with thecoordinate positions of the correction marks having been read, and whenthere are discrepancies between said two kinds of coordinate positions,to carry out said photographing after correcting an amount of movementof the holding members based on said amount of positional discrepancy ofthe board.

[0035] The inspection apparatus described in claim 7 of the presentinvention is an apparatus for inspecting a to-be-inspected board byphotographing the board that has been conveyed to an inspectionposition, in which a pair of holding members constituted of a pair ofrails for holding the board are moved to move the board in a directioncrossing a scanning direction of a photography unit and the board thathas been positioned is scanned by the photography unit along thedirection of said conveyance of the board, the inspection apparatuscomprising: a control unit configured to read coordinate positions ofcorrection marks applied to said board by the photography unit, tocompute an amount of positional discrepancy of the board by comparingcoordinate positions of correction marks previously set with thecoordinate positions of the correction marks having been read, and whenthere are discrepancies between said two kinds of coordinate positions,to carry out the photographing after correcting an amount of movement ofthe holding members based on said amount of positional discrepancy ofthe board.

BRIEF DESCRIPTION OF THE DRAWINGS

[0036]FIG. 1 is a schematic plan view of an inspection apparatus in afirst embodiment of the present invention;

[0037]FIG. 2 is a side elevation and front elevation of the inspectionapparatus in the first embodiment of the present invention;

[0038]FIG. 3 is a block diagram of the inspection apparatus in the firstembodiment of the present invention;

[0039]FIG. 4 is a flowchart showing an inspection method in the firstembodiment of the present invention;

[0040]FIG. 5 is a schematic plan view of an inspection apparatus in asecond embodiment of the present invention;

[0041]FIG. 6 is a flowchart showing an inspection method in the secondembodiment of the present invention;

[0042]FIG. 7 is a perspective view of a conventional inspectionapparatus;

[0043]FIG. 8 is a perspective view of a main part of the conventionalinspection apparatus;

[0044]FIG. 9 is a schematic plan view of the conventional inspectionapparatus; and

[0045]FIG. 10 is a schematic illustration showing steps in aconventional inspection method.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0046] Embodiment aspects are now described with the use of FIGS. 1 to5.

[0047] In this description, the same or similar items as those indicatedin FIGS. 7 to 10 diagramming the conventional example, as noted earlier,are designated by the same symbols.

First Embodiment

[0048] First embodiment in the present invention is shown in FIGS. 1 to4.

[0049] This first embodiment, as shown in FIG. 1, differs from theconventional example described earlier in that a screw shaft (ball screwshaft) 4 is provided for moving a printed circuit board 1, that is aboard being inspected, in a direction crossing the direction ofconveyance of the board (the direction indicated by arrow C), forexample, in the breadth direction of a pair of rails 2 indicated byarrow B, and correction marks for correcting the amount of rotationaldrive of the screw shaft 4 are provided on the side of the rails 2.

[0050] More specifically, as shown in FIGS. 2a and 2 b, the inspectionapparatus 40 is comprised of two movement shafts, namely the screw shaft4 for moving the printed circuit board 1 which has been conveyed to aninspection position, along the breadth direction of the rails 2(direction indicated by arrow B), and a movement shaft 8 for moving aline sensor camera 3 or a photography unit along the direction ofconveyance of the board (direction indicated by arrow A).

[0051] Here, 9 represents a moving platform for mounting thereon thepair of rails 2, which is driven to move in the direction indicated byarrow B as the screw shaft 4 rotates; 20 a motor for rotationallydriving the screw shaft 4; 21 a rotary encoder for detecting an amountof rotation of the motor 20; 22 a motor for rotationally driving themovement shaft 8; 23 a rotary encoder for detecting an amount ofrotation of the motor 22; 25 a control unit for controlling the overallinspection apparatus including the motors 20 and 22; and 30 an imageprocessing unit for processing image data photographed by the linesensor camera 3.

[0052] The control system of the inspection apparatus 40 is configuredas shown in FIG. 3.

[0053] In FIG. 3, the control unit 25 is connected with an input device26 and CRT 31, so that an operator operates the inspection apparatus 40by inputting instructions, information data and so on to the inputdevice 26 while watching the CRT 31. Represented by 32 is a storage unitfor storing therein data input from the input device 26, controlprograms necessary to actuate the inspection apparatus 40, controlparameters, image data photographed by the line sensor camera 3 and soon.

[0054] The control unit 25 rotationally drives the motor 20 for movingthe board, and the amount of rotation of the motor 20 is detected by theencoder 21. Likewise, the control unit 25 rotationally drives the motor22 for moving the line sensor camera, and the amount of rotation of themotor 22 is detected by the encoder 23.

[0055] The printed circuit board 1 is held by the pair of rails 2constituting a pair of holding members, and first and second correctionmarks 6 a and 6 b for the holding members are provided on the side 51 ofthe rails 2 where the line sensor camera 3 starts reading.

[0056] The screw shaft 4 sometimes expands or contracts due to heat andother factors, with the passage of time, so that discrepancies can occurbetween coordinate positions where the first and second holding membercorrection marks 6 a and 6 b have been recognized on the rails 2 at apoint of time t1, and coordinate positions where the first and secondholding member correction marks 6 a and 6 b have been recognized on therails 2 at another point of time t2 after a passage of a certain periodof time from t1.

[0057] As a result, a positional discrepancy occurs at the first scanstart position 5 a and the second scan start position 5 b, respectively,and a discrepancy occurs in the first and second inspection areas 7 aand 7 b.

[0058] Thereupon, in this first embodiment, the coordinate positions ofthe first and second holding member correction marks 6 a and 6 b areread by the line sensor camera 3, and the control illustrated by theflow chart shown in FIG. 4 is performed by the control unit 25 so thatthe amount of movement of the printed circuit board 1 in the directionindicated by arrow B is corrected.

[0059] First, in step S1, the coordinate positions of the first andsecond correction marks 6 a and 6 b are set by the input device 26 andstored in the storage unit 32.

[0060] In step S2, the motor 20 is driven to move the printed circuitboard 1 in the direction of B in order to photograph the first scan areaof the printed circuit board 1.

[0061] In step S3, every time one printed circuit board 1 is inspected,or once every several boards have been inspected, or when a valuemeasured by a screw shaft temperature detector 35 for detectingtemperature of the screw shaft 4 exceeds a previously set thresholdvalue, the first correction mark 6 a provided on one of the rails 2constituting the holding members is recognized, and in step S4,according to the instruction from the control unit 25, the line sensorcamera 3 is moved in the direction A to photograph the first holdingmember correction mark 6 a, and thus photographed correction mark issubjected to image processing by the image processing unit 30 so thatthe position of the first holding member correction mark 6 a isdetected. Then, the amount of positional discrepancy between thecoordinate position of the first holding member correction mark 6 adetected by photography and that of the set holding member correctionmark 6 a is computed.

[0062] When it is judged in step S5 that there is a discrepancy betweenthe coordinate positions of the two kinds of the first correction marks6 a, the amount of movement of the printed circuit board 1 is correctedand the positional discrepancy of the first scan start position 5 a iscorrected in step S6.

[0063] Specifically, the amount of rotation of the motor 22 is detectedby the encoder 23 in order that the motor 22 rotates for the same amountcorresponding to the amount of positional discrepancy obtained in stepS4, thereby correcting the amount of positional discrepancy and solvingthe problem associated with such discrepancy.

[0064] After the correction, the amount of coordinate positionaldiscrepancy of the first holding member correction mark is detectedagain in steps S3 and S4, and in step S5, it is determined whether ornot there exists the positional discrepancy. This is repeated until itis determined in step S5 that there exists no more positionaldiscrepancy.

[0065] When it is judged in step S5 that there is no positionaldiscrepancy, in step S7, the line sensor camera 3 is moved, for example,from a reading start side 51 toward a reading end side 52 as shown inFIG. 1, in order to photograph the first scan area 7 a of the printedcircuit board 1 by the line sensor camera 3.

[0066] At the same time, the first correction mark 6 c provided on theprinted circuit board 1 is photographed, and its center position isobtained by image processing by the image processing unit 30.

[0067] When it is judged in step S5 that there is no positionaldiscrepancy, too, the printed circuit board 1 is photographed in stepS7.

[0068] Next, in step S8, according to the instruction from the controlunit 25, the motor 20 is rotationally driven for an amount of movementcorresponding to the distance from the previously set first holdingmember correction mark 6 a to the second holding member correction mark6 b, and the printed circuit board 1 is moved in the direction B to aposition where the second scan area is photographed.

[0069] In step S9, according to the instruction from the control unit25, the line sensor camera 3 is moved in the direction A, and the secondholding member correction mark 6 b is photographed. And in step S10, theamount of positional discrepancy between the coordinate position of thesecond holding member correction mark 6 b which has been read and thecoordinate position of the previously set second holding membercorrection mark 6 b is computed.

[0070] When it is judged in step S11 that there is a coordinatepositional discrepancy with the second holding member correction mark 6b, in the same manner as in step S6, the amount of movement of theprinted circuit board 1 is corrected and the positional discrepancy ofthe first scan start position 5 a is corrected. Such corrections arerepeated in steps S9 and S10, until it is judged again that there is nopositional discrepancy in step S11.

[0071] When it is judged that there is no positional discrepancy in stepS11, the second scan area 7 b of the printed circuit board 1 isphotographed by the line sensor camera 3 in step S13. At the same time,the board correction mark 6 d on the printed circuit board 1 is alsophotographed, and its center position is obtained through imageprocessing by the image processing unit 30.

[0072] Next, in step S14, the amount of discrepancies between therecognized coordinate positions of the first and second board correctionmarks 6 c and 6 d obtained in step S7 and step S13, and the coordinatepositions of the board correction marks 6 c and 6 d previously set iscomputed, and in order to solve the discrepancy amount, the image of theprinted circuit board including the first scan area and the second scanarea combined is corrected by employing the conventional method earliermentioned so that the board correction marks 6 c and 6 d appearing onthe image correspond to the previously set positions.

[0073] In step S15, the corrected image of the printed circuit board issubjected to image processing and inspection judgment, thereafter theresult of inspection is output.

[0074] By performing such correction as described above, positionalcorrections can be performed to compensate for the expansion andcontraction in movement shafts and in printed circuit boards caused byheat and other factors, even in inspection machines that are configuredwith biaxial movement shafts.

[0075] In the foregoing description, the first and second correctionmarks 6 a and 6 b are placed in an opposed relations to each other onthe rails on the side where the line sensor camera 3 starts reading, butthe present invention is not limited thereto or thereby, and their setpositions can be adjusted as may be expedient.

Second Embodiment

[0076] Second embodiment of the present invention is shown in FIGS. 5and 6.

[0077] In this embodiment, although there is a difference from the firstembodiment described in the foregoing in that correction marks areprovided on a printed circuit board 1, in other respects the basicconfiguration is the same as in the first embodiment.

[0078] Specifically, as shown in FIG. 5, a first board correction mark 6c is provided on a printed circuit board 1 on the side where a linesensor camera 3 starts reading the printed circuit board 1 as shown inFIG. 9 which shows a conventional example, and a second board correctionmark 6 d is provided on a diagonal line passing the first correctionmark 6 c.

[0079] The printed circuit board 1 conveyed to the inspection positionis moved by a screw shaft 4 in the direction indicated by arrow B, andthe line sensor camera 3 is moved in the direction indicated by arrow A,whereby a first scan area 7 a and a second scan area 7 b are subjectedto inspection.

[0080] In an inspection apparatus configured in this way, when expansionor contraction occurs in the screw shaft 4 over the course of time dueto heat and/or other factors, and a discrepancy occurs between theposition obtained as a result of recognition of the board correctionmarks 6 c and 6 d on the printed circuit board 1 at some point of timeand the position obtained as a result of recognition of the boardcorrection marks 6 c and 6 d on the printed circuit board 1 after apassage of some period of time, the position of the printed circuitboard 1 is corrected by the control unit shown in FIG. 6.

[0081] First, in step S1, the coordinate positions of the first andsecond board correction marks 6 c and 6 d are set by the input device 26and stored in the storage unit 32.

[0082] In step S2, the motor 20 is driven to move the printed circuitboard 1 in the direction of B in order to photograph the first scan areaof the printed circuit board 1.

[0083] In step S3, every time one printed circuit board 1 is inspected,or once every several boards have been inspected, or when a valuemeasured by a screw shaft temperature detector 35 for detectingtemperature of the screw shaft 4 exceeds a previously set thresholdvalue, the first board correction mark 6 c provided on the printedcircuit board is recognized, and in step S4, according to theinstruction from the control unit 25, the line sensor camera 3 is movedin the direction A to photograph the first board correction mark 6 c,and thus photographed correction mark is subjected to image processingby the image processing unit 30 so that the position of the first boardcorrection mark 6 c is detected. Then, the amount of positionaldiscrepancy between the coordinate position of the first boardcorrection mark 6 c detected by photography and that of the set boardcorrection mark 6 c is computed.

[0084] When it is judged in step S5 that there is a discrepancy betweenthe coordinate positions of the two kinds of the first board correctionmarks 6 c, the amount of movement of the printed circuit board 1 iscorrected and the positional discrepancy of the first scan startposition 5 a is corrected in step S6.

[0085] Specifically, the amount of rotation of the motor 22 is detectedby the encoder 23 in order that the motor 22 rotates for the same amountcorresponding to the amount of positional discrepancy obtained in stepS4, thereby correcting the amount of positional discrepancy and solvingthe problem associated with such discrepancy.

[0086] After the correction, the amount of coordinate positionaldiscrepancy of the first board correction mark is detected again insteps S3 and S4, and in step S5, it is determined whether or not thereexists the positional discrepancy. This is repeated until it isdetermined in step S5 that there exists no more positional discrepancy.

[0087] When it is judged in step S5 that there is no positionaldiscrepancy, in step S7, the line sensor camera 3 is moved, for example,from a reading start side 51 toward a reading end side 52 as shown inFIG. 5, in order to photograph the first scan area 7 a of the printedcircuit board 1 by the line sensor camera 3.

[0088] At the same time, the first board correction mark 6 c provided onthe printed circuit board 1 is photographed, and its center position isobtained by image processing by the image processing unit 30.

[0089] When it is judged in step S5 that there is no positionaldiscrepancy, too, the printed circuit board 1 is photographed in stepS7.

[0090] Next, in step S8, according to the instruction from the controlunit 25, the motor 20 is rotationally driven for an amount of movementin the direction B corresponding to the distance from the previously setfirst board correction mark 6 c to the second board correction mark 6 d,and the printed circuit board 1 is moved in the direction B to aposition where the second scan area is photographed.

[0091] In step S9, according to the instruction from the control unit25, the line sensor camera 3 is moved in the direction A, and the secondboard correction mark 6 d is photographed. And in step S10, the amountof positional discrepancy between the coordinate position of the secondboard correction mark 6 d which has been read and the coordinateposition of the previously set second board correction mark 6 d iscomputed.

[0092] When it is judged in step S11 that there is a coordinatepositional discrepancy with the second board correction mark 6 d, in thesame manner as in step S6, the amount of movement of the printed circuitboard 1 is corrected and the positional discrepancy of the first scanstart position 5 a is corrected. Such corrections are repeated in stepsS9 and S10, until it is judged again that there is no positionaldiscrepancy in step S11.

[0093] When it is judged that there is no positional discrepancy in stepS11, the second scan area 7 b of the printed circuit board 1 isphotographed by the line sensor camera 3 in step S13. At the same time,the board correction mark 6 d on the printed circuit board 1 is alsophotographed, and its center position is obtained through imageprocessing by the image processing unit 30.

[0094] Next, in step S14, the amount of discrepancies between therecognized coordinate positions of the first and second board correctionmarks 6 c and 6 d obtained in step S7 and step S13, and the coordinatepositions of the board correction marks 6 c and 6 d previously set iscomputed, and in order to solve the discrepancy amount, the image of theprinted circuit board including the first scan area and the second scanarea combined is corrected by employing the conventional method earliermentioned so that the board correction marks 6 c and 6 d appearing onthe image correspond to the previously set positions.

[0095] In step S15, the corrected image of the printed circuit board issubjected to image processing and inspection judgment, thereafter theresult of inspection is output.

[0096] By performing such correction as described above, positionalcorrections can be performed to compensate for the expansion andcontraction in movement shafts and in printed circuit boards due to heatand/or other factors, even in inspection machines that are configuredwith two movement shafts as in the first embodiment described earlier.

[0097] In the foregoing description, the positional discrepancy of thescrew shaft is corrected using the same first and second correctionmarks as used in the conventional expansion and contraction correctionmethod, but the present invention is not limited thereto or thereby, noris there any particular limitation on the positions of the first andsecond board correction marks used in the correction shown in FIG. 5.

[0098] In the embodiments mentioned above, description has been made byciting a ball screw as an element constituting the movement shaft, butthe present invention is not limited thereto or thereby, and can beembodied with some other drive means such as a timing belt or the like.

[0099] As is clear from the embodiments mentioned above, according tothe inspection method of the present invention, the coordinate positionsof correction marks applied to holding members are read by a line sensorcamera, an amount of positional discrepancy of a board is computed bycomparing previously set coordinate positions of the correction markswith coordinate positions of the read correction marks, and when thereare discrepancies between the two kinds of coordinate positions,photographing noted earlier is performed after correcting an amount ofmovement of the holding members based on the amount of positionaldiscrepancy of the board, whereby it is possible to inspect a large-sizeboard by laterally moving the board and photographing the same for aplural number of times before and after the lateral moving, and it isalso possible to achieve high-precision inspection.

[0100] Further, with the inspection apparatus of the present invention,the inspection method of the present invention can be easily implementedby providing a control unit configured so that the coordinate positionsof the correction marks applied to the holding members are read by theline sensor camera, an amount of positional discrepancy of the board iscomputed by comparing the coordinate positions of the previously setcorrection marks with the coordinate positions of the read correctionmarks, and when there are coordinate-positional discrepancies betweenthe two kind of correction marks, the photographing is performed aftercorrecting the amount of movement of the holding members based on theamount of positional discrepancy of the board.

What is claimed is:
 1. An inspection method for inspecting a to-be-inspected board by photographing the board that has been conveyed to an inspection position, in which a pair of holding members for holding the board are moved to move the board in a direction crossing a scanning direction of a photography unit and the board that has been positioned is scanned by the photography unit along the direction of said conveyance of the board, the inspection method comprising: reading coordinate positions of holding member correction marks applied to said holding members by said photography unit; computing an amount of positional discrepancy of the board by comparing coordinate positions of holding member correction marks previously set with the coordinate positions of holding member correction marks having been read; and when there are discrepancies between said two kinds of coordinate positions, carrying out said photographing after correcting an amount of movement of the holding members based on said amount of positional discrepancy of the board.
 2. An inspection method for inspecting a to-be-inspected board by photographing the board that has been conveyed to an inspection position, in which a pair of holding members constituted of a pair of rails located in an opposed relation to each other for holding the board are moved to move the board in a direction crossing a scanning direction of a photography unit and the board that has been positioned is scanned by the photography unit along the direction of said conveyance of the board, the inspection method comprising: reading a coordinate position of a first holding member correction mark applied to one of the holding members by said photography unit; moving said holding members and reading a coordinate position of a second holding member correction mark applied to the other of the holding members, said second holding member correction mark on said the other of the holding members being on the side opposite to that of said first holding member correction mark on said one of the holding members with the board being interposed therebetween; computing an amount of positional discrepancy of the board by comparing coordinate positions of first and second holding member correction marks previously set with the coordinate positions of the first and second holding member correction marks having been read; and when there are discrepancies between said two kinds of coordinate positions, carrying out said photographing after correcting an amount of movement of the holding members based on said amount of positional discrepancy of the board.
 3. An inspection method for inspecting a to-be-inspected board by photographing the board that has been conveyed to an inspection position, in which a pair of holding members for holding the board are moved to move the board in a direction crossing a scanning direction of a photography unit and the board that has been positioned is scanned by the photography unit along the direction of said conveyance of the board, the inspection method comprising: reading coordinate positions of board correction marks applied to said board by said photography unit; computing an amount of positional discrepancy of said board by comparing coordinate positions of board correction marks previously set with the coordinate positions of the board correction marks having been read; and when there are discrepancies between said two kinds of coordinate positions, carrying out said photographing after correcting an amount of movement of the holding members based on said amount of positional discrepancy of the board.
 4. The inspection method according to claim 2, wherein the first and second holding member correction marks are provided on the side where the photography unit starts reading.
 5. The inspection method according to claim 3, wherein a first board correction mark and a second board correction mark are provided on the board, said first board correction mark being provided on the side where the photography unit starts reading, said second board correction mark being provided on a diagonal line passing said first board correction mark.
 6. An inspection apparatus for inspecting a to-be-inspected board by photographing the board that has been conveyed to an inspection position, in which a pair of holding members for holding the board are moved to move the board in a direction crossing a scanning direction of a photography unit and the board that has been positioned is scanned by the photography unit along the direction of said conveyance of the board, the inspection apparatus comprising: a control unit configured to read coordinate positions of holding member correction marks applied to the holding members by the photography unit, to compute an amount of positional discrepancy of the board by comparing coordinate positions of holding member correction marks previously set with the coordinate positions of the holding member correction marks having been read, and when there are discrepancies between said two kinds of coordinate positions, to carry out said photographing after correcting an amount of movement of the holding members based on said amount of positional discrepancy of the board.
 7. An inspection apparatus for inspecting a to-be-inspected board by photographing the board that has been conveyed to an inspection position, in which a pair of holding members constituted of a pair of rails for holding the board are moved to move the board in a direction crossing a scanning direction of a photography unit and the board that has been positioned is scanned by the photography unit along the direction of said conveyance of the board, the inspection apparatus comprising: a control unit configured to read coordinate positions of board correction marks applied to said board by the photography unit, to compute an amount of positional discrepancy of the board by comparing coordinate positions of board correction marks previously set with the coordinate positions of the board correction marks having been read, and when there are discrepancies between said two kinds of coordinate positions, to carry out the photographing after correcting an amount of movement of the holding members based on said amount of positional discrepancy of the board. 